A statement purporting to come from the Movement for the Emancipation of the Niger Delta said they initially sabotaged the undersea pipeline on March 1 and that scuba divers early Thursday caused "further damage to the ongoing repair works."
Shell Nigeria did not immediately respond to requests for comment, and there was no way to independently verify the militants' claim.
On Tuesday, Shell blamed "unknown persons (who) installed a crude theft point" for the leak on the pipe eight meters (26 feet) below sea level. The company, part-owned by Nigeria's government, declared "force majeure" effective Tuesday to get some legal protection against contractual obligations.
Shell has refused to say how much oil has not been exported since it halted Forcados exports on March 4. The terminal can handle 400,000 barrels of crude daily, more than a fifth of Nigeria's production of 2.2 billion barrels.
Sabotage by militants largely ended with a 2009 government amnesty that bought out militant leaders and more than 26,000 fighters, some of whom were contracted to guard the very installations they had been attacking. But some militants who did not benefit from the amnesty, and others who have since lost contracts, recently have threatened more attacks.
At the height of the insurgency, militants halted a quarter of Nigeria's oil output with attacks on facilities and kidnappings of foreign oil workers.
Since then, oil theft attributed to other causes has risen to more than 200,000 barrels a day, most blamed on large criminal enterprises who sell the crude on the oil markets in deals that also benefit politicians and senior military officers. All those thefts occur on land.
President Goodluck Jonathan this week announced the government has earmarked $1 billion to fight oil theft.
Most communities in the oil-rich delta are impoverished despite decades of oil production that should make Nigerians wealthy but benefits only an elite.
The Niger Delta militants claim to be fighting for justice for such ordinary people.
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